The rotator cuff comprises four muscles that control shoulder movement and their associated tendons by which the muscles are attached to the humeral head (the end of the humerus that forms part of the shoulder joint). Together with the joint capsule, these muscles and tendons form a cuff that surrounds the head of the humerus. The tendons are thick, ribbon-like structures having high tensile strength. They not only hold the shoulder muscles and the humerus together, but also transmit forces exerted by those muscles to induce corresponding movement of the humerus. In a healthy subject, the tendons are fully attached at one end to, and envelop, a portion of the humeral-head surface. When a rotator cuff tear occurs, the joint capsule and one or more of the tendons become(s) partially or entirely torn away from the humeral head, creating both a painful and a functionally debilitating condition.
Current treatment for rotator-cuff tears is to suture the torn tendon back to the bone of the humeral head. The sutures hold the tendon in contact with the bone, preferably long enough for the tendon to heal to the bone and form a bridge that will re-establish the tendon-bone connection and restore normal function. The sutures that are used possess sufficient tensile strength to retain the tendon and bone together during the healing process. However, the tendon is a fibrous tissue that can be torn by the sutures. Commonly, the sutures will align with the fascicular structure of the tendon and tear right through it under sufficient tensile force, thus undoing the surgical repair before tendon-to-bone healing is complete. The sutures can also tear through the bone under sufficient force, particularly in older subjects who form the bulk of rotator-cuff-tear patients and whose bones tend to be more osteoporotic.
In fact, rotator-cuff tears affect 40% or more of those over age 60 and cost the US economy approximately $3 billion per year. The repair failure rate of large to massive rotator cuff tears ranges from 20 to 90%. High re-tear rates are a result of mechanical factors (e.g., tear size, repair technique, rehabilitation protocol) as well as biologic factors (e.g., age, tear chronicity, tendon quality, disease) that may compromise the patients' intrinsic capacity to heal. All of these factors may also contribute to the propensity of the sutures to tear through the tendon and bone before healing is complete, thus contributing to the re-tear rate. Hence, there is a need for repair strategies that provide adequate strength as well as stimulate and enhance healing potential.